maria-developers team mailing list archive

[Jan Lindström <jan.lindstrom@xxxxxxxxxxx>]

Hi Sergei,

After QA runs done by Ramesh, we now know the latest fix candidate i.e.
what is in bb-10.2-MDEV-25114-galera-v2 is incorrect. Problem is in
wsrep_close_connections() as it holds LOCK_thread_count while it does
abort_replicated that will call wsrep_abort_transaction and there we use
find_thread_by_id that would also take LOCK_thread_count. As there is
another code path here, the problem is not easily fixed. We can't just
release LOCK_thread_count at wsrep_close_connections as we iterate the
thread list.

I must say I'm not sure what to do now.

(gdb) bt
#0  __pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at
../sysdeps/unix/sysv/linux/pthread_kill.c:56
#1  0x000056333963a2e8 in my_write_core (sig=sig@entry=6) at
/test/mtest/10.2_dbg/mysys/stacktrace.c:382
#2  0x0000563338f2993d in handle_fatal_signal (sig=6) at
/test/mtest/10.2_dbg/sql/signal_handler.cc:355
#3  <signal handler called>
#4  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#5  0x000014b799572859 in __GI_abort () at abort.c:79
#6  0x000056333963edc4 in safe_mutex_lock (mp=0x563339e47220
<LOCK_thread_count>, my_flags=my_flags@entry=0,
    file=file@entry=0x5633396d9050 "/test/mtest/10.2_dbg/sql/sql_parse.cc",
line=line@entry=8902)
    at /test/mtest/10.2_dbg/mysys/thr_mutex.c:264
#7  0x0000563338d1aea7 in inline_mysql_mutex_lock (src_line=8902,
src_file=0x5633396d9050 "/test/mtest/10.2_dbg/sql/sql_parse.cc",
    that=<optimized out>) at
/test/mtest/10.2_dbg/include/mysql/psi/mysql_thread.h:688
#8  find_thread_by_id (id=id@entry=48, query_id=query_id@entry=false) at
/test/mtest/10.2_dbg/sql/sql_parse.cc:8902
#9  0x0000563339134c39 in wsrep_abort_transaction (hton=<optimized out>,
bf_thd=0x14b680000d90, victim_thd=<optimized out>,
    signal=<optimized out>) at
/test/mtest/10.2_dbg/storage/innobase/handler/ha_innodb.cc:19821
#10 0x0000563338f38cbf in ha_abort_transaction
(bf_thd=bf_thd@entry=0x14b680000d90,
victim_thd=victim_thd@entry=0x14b680000d90,
    signal=signal@entry=1 '\001') at
/test/mtest/10.2_dbg/sql/handler.cc:6327
#11 0x0000563338ebed6d in wsrep_abort_thd
(bf_thd_ptr=bf_thd_ptr@entry=0x14b680000d90,
victim_thd_ptr=victim_thd_ptr@entry=0x14b680000d90,
    signal=signal@entry=1 '\001') at
/test/mtest/10.2_dbg/sql/wsrep_thd.cc:832
#12 0x0000563338eaa2bd in abort_replicated (thd=thd@entry=0x14b680000d90)
at /test/mtest/10.2_dbg/sql/wsrep_mysqld.cc:2269
#13 0x0000563338eae097 in wsrep_close_client_connections
(wait_to_end=wait_to_end@entry=1 '\001',
    except_caller_thd=except_caller_thd@entry=0x0) at
/test/mtest/10.2_dbg/sql/wsrep_mysqld.cc:2437
#14 0x0000563338eaedf6 in wsrep_stop_replication (thd=thd@entry=0x0) at
/test/mtest/10.2_dbg/sql/wsrep_mysqld.cc:962
#15 0x0000563338c543d8 in kill_server (sig_ptr=sig_ptr@entry=0x0) at
/test/mtest/10.2_dbg/sql/mysqld.cc:2009
#16 0x0000563338c558d5 in kill_server_thread (arg=<optimized out>) at
/test/mtest/10.2_dbg/sql/mysqld.cc:2047
#17 0x000014b799a7a609 in start_thread (arg=<optimized out>) at
pthread_create.c:477
#18 0x000014b79966f293 in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:95

R: Jan

On Wed, Oct 6, 2021 at 5:03 PM Sergei Golubchik <serg@xxxxxxxxxxx> wrote:

> Hi, Jan!
>
> On Oct 06, Jan Lindström wrote:
> > >
> > > > > > +/* This is wrapper for wsrep_break_lock in thr_lock.c */
> > > > > > +static int wsrep_thr_abort_thd(void *bf_thd_ptr, void
> *victim_thd_ptr, my_bool signal)
> > > > > > +{
> > > > > > +  THD* victim_thd= (THD *) victim_thd_ptr;
> > > > > > +  /* We need to lock THD::LOCK_thd_data to protect victim
> > > > > > +  from concurrent usage or disconnect or delete. */
> > > > >
> > > > > How do you know victim_thd wasn't deleted before you locked
> > > > > LOCK_thd_data below?
> > > >
> > > > I must say the thr_lock code is not familiar to me but there are
> > > > mysql_mutex_lock() calls to lock->mutex. After code review  it is not
> > > > clear to me what that mutex is.
> > >
> > > where are mysql_mutex_lock() calls to lock->mutex?
> >
> > mysys/thr_lock.c there is function thr_lock() there is call to
> > mysql_mutex_lock(&lock->mutex); this is before wsrep_break_lock where we
> > call wsrep_thd_abort
>
> this is for table locks. `lock` is `data->lock` where `data` is THR_LOCK
> structure somewhere in the table share. It locks tables and handlers,
> not threads. And InnoDB isn't using it at all anyway.
>
> >
> > > > > >          if (victim_trx) {
> > > > > > +                wsrep_thd_UNLOCK(victim_thd);
> > > > >
> > > > > what keeps victim_trx from disappearing here?
> > > >
> > > > Nothing. Do you have suggestions ?
> > >
> > > A couple of thoughts:
> > >
> > > * Why do you unlock LOCK_thd_data here at all? I believed the whole
> > > point of using TOI was to make sure that even if you lock mutexes in
> > > a different order, it will not cause a deadlock.
> >
> > This is DDL-case when we have a MDL-conflict not user KILL, we need to
> > release it in my opinion because we need to take mutexes in
> > lock_sys -> trx -> THD::LOCK_thd_data order,
> > if I do not release I can see easily safe_mutex warnings
>
> I don't understand. First, lock_sys and trx mutexes are not covered by
> safe_mutex, so it cannot possibly warn about them.
>
> Second, the reason for taking mutexes always in the same order is to
> avoid deadlocks. And yor TOI trick should archieve that.
>
> > > * You cannot pass a THD safely over a gap where no locks keep it in
> > > existence. You can pass an integer, thread id, and use
> > > find_thread_by_id later.
> >
> > I have a suggestion for this:
> >
> >          if (victim_trx) {
> > +
> > +               const trx_id_t victim_trx_id= victim_trx->id;
> > +               const longlong victim_thread=
> thd_get_thread_id(victim_thd);
> >                 lock_mutex_enter();
> > -               trx_mutex_enter(victim_trx);
> > -               wsrep_innobase_kill_one_trx(bf_thd, bf_trx, victim_trx,
> signal);
> > +               if (trx_t* victim= trx_rw_is_active(victim_trx_id, NULL,
> true)) // If this succeeds, trx can not go away
> > +               {
> > +                       trx_mutex_enter(victim);
> > +                       ut_ad(victim->mysql_thd ?
> > +                               thd_get_thread_id(victim->mysql_thd) ==
> victim_thread :
> > +                               1);
> > +                       wsrep_innobase_kill_one_trx(bf_thd, bf_trx,
> victim, signal);
> > +                       trx_mutex_exit(victim);
> > +                       victim->release_reference(); // Now trx can go
> away after we have released lock_sys
> > +                       wsrep_srv_conc_cancel_wait(victim);
> > +               }
> >                 lock_mutex_exit();
> > -               trx_mutex_exit(victim_trx);
> > -               wsrep_srv_conc_cancel_wait(victim_trx);
> >                 DBUG_VOID_RETURN;
>
> I don't understand, sorry. It's not quite clear what this patch should
> apply to. May be you can paste what the new code should look like, not a
> patch?
>
> In particular, where are wsrep_thd_UNLOCK and wsrep_thd_LOCK. Are they
> present in this new variant at all?
>
> >
> > > By the way, how long can WSREP_TO_ISOLATION_BEGIN() take?
> > > Does it need to wait for everything else being replicated?
> >
> > As long as it takes to start it on all nodes in the cluster.
>
> So, KILL basically won't work when a cluster is starting.
> Can bf aborts happen during a cluster startup?
>
> Regards,
> Sergei
> VP of MariaDB Server Engineering
> and security@xxxxxxxxxxx
>